Clean, wash and cut the rhubarb into large pieces. Put 400 g rhubarb aside. Put the remaining rhubarb into a juicer and extract the juice. Measure 700 ml rhubarb juice. Cut the rhubarb aside in the meantime into smaller pieces. Mix with the rhubarb juice and jam sugar in a tall pot.
Bring to the boil while stirring. Cook for 4 minutes until bubbly. Stir in Campari. Skim off any foam that may have formed. Immediately pour into carefully prepared glasses and close well